The invention relates to the production of a solid-state battery, and more particularly to the joint between an anode and a cathode via a non-electron-conducting electrolyte layer.
Rechargeable lithium-ion batteries, hereafter also referred to as Li-ion batteries, have had widespread success in recent years. They can already be found in many mobile devices. In addition to hybrid and electric vehicles, their field of application also includes the potential storage of power from wind or solar energy plants. However, today's Li-ion batteries cannot yet satisfy some requirements, which is why many efforts are being undertaken to explore novel battery concepts.
Electrochemical storage devices that are based on the galvanic cell, which are colloquially also referred to as batteries, are particularly suitable for storing electrical energy. They are always composed of an anode, a cathode, and an electrolyte. Batteries can be intended for one-time use (primary battery) or be reusable (secondary battery, rechargeable battery).
There are a multitude of possible materials for the anode and the cathode that can be used for classification, the best-known arguably being zinc-carbon (Zn—C) batteries, alkali-manganese (alkaline) batteries, nickel-cadmium (Ni—Cd) batteries, nickel-metal hydride (Ni-MH) batteries, and lead (Pb) batteries.
In addition, the electrolyte may also be used for classification, for example liquid, gel or polymer electrolytes can be used in Li-ion batteries. Current standard battery types contain liquid or gel-like electrolytes almost exclusively.
These liquid or gel-like electrolytes can pose considerable safety risks. They can be strong acids or lyes, for example, or contain short-chain solvents, which are easily flammable. In the event of heating, for example as a result of a short circuit, electrolyte may leak and jeopardize the user or ignite the entire battery. In Li-ion batteries, for example, this is known as thermal runaway and has created considerable problems for the large-scale use of this battery type not only in cars, but also in aircraft.
The solid-state battery or solid-state electrolyte battery type represents a new kind of battery that is inherently free of the aforementioned safety risks and is therefore clearly superior to conventional types in this regard. Instead of a (gel) electrolyte that is normally liquid or stabilized by way of polymers, these use an ion conducting solid material. This may be either organic (polymers, and the like) or inorganic (ceramics, glasses, and the like). Crucial aspects for the functionality of an electrolyte are low electronic conductivity, with high ionic conductivity and (electro)chemical stability with respect to the anode and cathode materials.
Examples of inorganic solid-state ion conductors are, for example, yttria-stabilized zirconia and gadolinium-stabilized coria, which are able to conduct oxygen ions and can be used as an electrolyte for high-temperature metal-metal oxide batteries having a composition similar to a solid oxide fuel cell. Further examples are β-alumina, which is able to conduct sodium ions and is used in Na liquid metal batteries, and lithium phosphorus oxynitride (UPON), which is able to conduct lithium ions and is used in lithium thin film batteries. This series can be continued for a multitude of further ions of elements or compounds (F−, CO2−, and the like) which could be used for potential battery applications and is therefore not exhaustive, and does not represent an exclusion criterion. This is only intended to point out the basic properties of the electrolyte material.
In research, for example, an intrinsically safe lithium solid-state battery comprising Al and Cu contacts, a LiFePO4 cathode, a Li7La3Zr2O12 electrolyte, and a Si anode is known. A thin film battery having a similar composition is also known and is already being commercialized, which is sold by Infinite Power Solutions under the trade name Thinergy®. High-temperature sodium batteries comprising the above-mentioned alumina solid electrolyte are already being sold by Fiamm Sonick for use in electric vehicles.
Typically, batteries in general comprise a single- or multi-phase mixture, as do the cathode and the anode in solid-state batteries, regardless of whether these are primary or secondary batteries. The phases are a first phase a, for example, comprising the active material for ion insertion or removal, a second phase b, comprising the material for ion conduction, and a further phase c, which comprises a material having electronic conductivity. Optionally, one or more functions may also be assumed by a material of another phase. The individual phases can be, but do not have to be, present in the same state and are hereafter referred to as mixed-conducting electrodes.
A conventional lithium-ion battery, for example, has a porous matrix made of carbon on the anode side, which is used both to insert the lithium ions and to transport the current. The cathode side has a porous matrix made of LiFePO4 and carbon. Both matrices are saturated with a liquid electrolyte. As a result, a two-phase mixture (solid-liquid) is present on the anode side, and a three-phase mixture (solid-solid-liquid) is present on the cathode side.
These anodes and cathodes are separated by a respective electrolyte, which represents a layer that is not conducting to electrons, but has the highest possible ionic conductivity. In the case of the conventional Li-ion battery, this is achieved by an additional porous, not electron- or ion-conducting separator that is saturated with electrolyte and used, among other things, to spatially separate the liquid-saturated anode and cathode. This spatial separation should advantageously be kept as thin as possible, in order to minimize the overall internal resistance of the cell.
Without being limited to these, typical methods for producing or applying a solid-state electrolyte are the methods listed below:
producing thin layers by way of typical ceramic methods (such as screen printing, tape casting, ink jet printing), or by way of physical or chemical thin film methods (such as physical or chemical vapor deposition, sol gel methods (dip coating, spin coating, and the like)) or else by way of thermal spraying (vacuum or atmospheric plasma spraying, and the like). These methods are all based on the application of an additional electrolyte layer having the above-mentioned properties, low electronic conductivity and high ionic conductivity, which may pose greater demands on process control, material selection and quality of the anodes, cathodes and electrolyte and may increase the technical complexity, and thus the costs of the production process and of the end product.
Exemplary embodiments of such solid-state batteries or batteries comprising solid-state electrolytes are known from industry, but so far have rather been a niche segment due to complex production methods and consequently high costs (per unit of capacity). A commercial lithium-based solid-state thin film cell is marketed by Infinite Power Solutions, for example, under the name Thinergy® MEC200. Every component of the cell is produced by way of a complex vapor deposition method. It is only possible to implement thin electrodes in this way, which drastically impairs the overall capacity of the cell.
Another exemplary embodiment is the “Zebra” battery marketed by FIAMM, which is a Na ion technology-based liquid-solid-liquid battery as described above and operated at approximately 270° C.
It is the object of the invention to provide a novel method for producing a solid-state battery which provides for an ion-conducting, but electrically insulating, layer that is as thin as possible, as the electrolyte between an anode and a cathode of a battery cell, while being simple and/or inexpensive.